A spark plug for use in a combustion apparatus, such as an internal combustion engine, includes, for example, a tubular insulator having an axial bore, a center electrode provided in an inserted manner at a forward end portion of the axial bore, a terminal electrode provided in an inserted manner at the rear side of the axial bore, and a tubular metallic shell provided externally of an outer circumference of the insulator. The terminal electrode is exposed from the rear end of the insulator and includes a head portion to which a plug cap or the like for supply of electricity is attached, and a rodlike leg portion whose forward end portion is fixed to the insulator by means of a glass seal layer or the like. Furthermore, the insulator includes a rear trunk portion provided at its rear side, exposed from the rear end of the metallic shell, and adapted to ensure electric insulation between the head portion and the metallic shell.
Additionally, the insulator is generally manufactured in the following manner. A material powder which contains alumina, etc., is compacted, yielding a green compact having a hole portion which is to become the axial bore. Next, a support pin is inserted into the hole portion of the green compact; then, a grinding, rotating roller is brought into contact with the outer circumferential surface of the green compact. The rotating roller grinds the green compact, thereby forming an insulator intermediate having substantially the same shape as that of the insulator; then, the insulator intermediate is fired, thereby yielding the insulator (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-210142).
Incidentally, in some cases, during operation of an internal combustion engine or the like, as a result of oscillation of the head portion of the terminal electrode with a forward end portion of the leg portion fixed to the insulator as a base point, the leg portion of the terminal electrode hits against the inner circumference of the rear trunk portion of the insulator. Hitting of the terminal electrode against the rear trunk portion may cause breakage of the rear trunk portion, or, even when the breakage is not reached, fine cracks may be formed in the rear trunk portion, resulting in deterioration in strength of the rear trunk portion. In view of prevention of breakage of the rear trunk portion and maintenance of strength of the rear trunk portion, increasing the wall thickness of the rear trunk portion for enhancement of strength is effective; however, in recent years, demand has arisen to reduce the size of a spark plug, requiring reduction in the diameter of the insulator. Thus, in order to achieve prevention of breakage of the rear trunk portion or a like problem while the insulator is reduced in diameter, imparting a relatively small inside diameter to the axial bore is conceived so as to increase the wall thickness (section modulus) of the rear trunk portion (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-100250).